Receptor-mediated endocytosis is the mechanism by which a variety of nutrients, hormones, and growth factors are specifically and efficiently transported into the cell as described in Goldstein et al, Annu. Rev. Cell Biol. 1:1-39 (1985). During this process, receptors are selectively concentrated in clathrin-coated pits from which they are rapidly internalized and delivered to endosomes; some receptors like the low density lipoprotein (LDL) receptor are constitutively clustered and internalized in the absence of ligand, while others, such as the epidermal growth factor (EGF) receptor are concentrated in the coated pits and internalized only after binding ligand, as described in Trowbridge, Current Opin. in Cell Biol. 3:634-641 (1991). The protein complexes that link clathrin to transmembrane proteins are called adaptors. Plasma-membrane adaptors contain an .alpha.-adaptin and a .beta.-adaptin subunit, while adaptors found in the Golgi apparatus contain a .gamma.-adaptin and a .beta.'-adaptin subunit, as described in Robinson, J. of Cell Biol. 111: 2319-2326 (1990).
Despite these general outlines of the process of receptor-mediated endocytosis, the identities of all of the molecules involved and the interactions among them are not known. Thus, there is a continuing need in the art for identification of components of this important process.